Chan Ho Kwon, Hong Lae Kim, and Myung Soo Kim*Chan Ho Kwon, Hong Lae Kim, and Myung Soo Kim*

National Creative Research Initiative Center for Control of Reaction National Creative Research Initiative Center for Control of Reaction Dynamics and School of Chemistry, Seoul National University, Dynamics and School of Chemistry, Seoul National University, Seoul 151-742, KoreaSeoul 151-742, Korea

Vibrational spectra of halobenzene cations in the ground Vibrational spectra of halobenzene cations in the ground

and and 22BB22 electronic states obtained by one-photon electronic states obtained by one-photon

mass-analyzed threshold ionization spectrometrymass-analyzed threshold ionization spectrometry

Vibrational spectra of halobenzene cations in the ground Vibrational spectra of halobenzene cations in the ground

and and 22BB22 electronic states obtained by one-photon electronic states obtained by one-photon

mass-analyzed threshold ionization spectrometrymass-analyzed threshold ionization spectrometry

B~

Contents

ⅠⅠ. . Motivation for researchMotivation for research

ⅡⅡ . Mass-analyzed threshold ionization (MAT. Mass-analyzed threshold ionization (MATII) spectroscopy) spectroscopy

ⅢⅢ. . MATMATII spectra in the ground electronic state spectra in the ground electronic state

ⅣⅣ. . MATMATII spectra in the spectra in the 22BB22 excited electronic state excited electronic state

ⅤⅤ. . Selection ruleSelection rule

ⅥⅥ. . Summary and conclusionSummary and conclusion

B~

ⅠⅠ. . Motivation for researchMotivation for research

A. Excited electronic states of polyatomic ionsA. Excited electronic states of polyatomic ions

Cases of very long-lived (‘metastable’) excited electronic states are very rare for polyatomic (n≥4) ions.

Decay mechanismsDecay mechanisms

( ) ⅰ Internal conversion to the ground electronic state

( ) ⅱ Dissociation on a repulsive electronic state

( ) ⅲ Radiative decay

Absolute prevalence of Absolute prevalence of ( )ⅰ( )ⅰ has led to the theory of mass has led to the theory of mass spectra (RRKM-QET)spectra (RRKM-QET)

‘Molecular ions undergo internal conversion to the ground state and dissociate statistically (RRKM or microcanonical transition state theory) there in’

B. Discovery of very long-lived excited electronicB. Discovery of very long-lived excited electronic

states of polyatomic ionsstates of polyatomic ions

1) 1) Charge exchange ionizationCharge exchange ionization

A+ + B → A + B+

E = IE(B) - RE(A+)E = IE(B) - RE(A+)

IE : Ionization energy

RE: Recombination energy of A+

= Ionization energy of A to the state in which A+ is in.

For charge exchange under near thermal condition involving

polyatomics, cross section is very large only when

E ≤ 0Exoergicity criterion’

2) 2) Halobenzene and related ions Halobenzene and related ions

Some electronic states of C6H5X+• ( X = Cl, Br, I )Some electronic states of C6H5X+• ( X = Cl, Br, I )

Ground state neutralGround state neutral

3b1 , 1a2 - e1g of benzene

6b2 - n(X3p∥)

2b1 - n(X3p⊥)

IIons ons

These are states appearing in photoelectron spectra.

3b1

1a2

6b2

2b1

121

1 BX~)b( -2

212 AA~)a( -

221

2 BB~)b( -1

211 BC

~)b( -

C6H5C≡N+• and C6H5C≡CH+•C6H5C≡N+• and C6H5C≡CH+•

Low – lying electronic states are similar to C6H5X+•

state - Loss of e- from (C≡N∥) or (C≡C∥)B~

state - Loss of e- from (C≡N⊥) or (C≡C⊥)C~

TABLE 1TABLE 1. Collision gases, their ionization energies(. Collision gases, their ionization energies(IIE) in eV, and success / failure E) in eV, and success / failure to generate their ions by charge exchange with some precursor ions to generate their ions by charge exchange with some precursor ions

Recombination energy( ) 9.066 8.991 9.71 8.75 8.754 9.20

Recombination energy( ) 11.330 10.633 11.84 10.36 9.771 12.24B~X~

Discovery

states of C6H5Cl+• , C6H5Br+• , C6H5CN+• , C6H5CCH+•

are very long – lived ( > 10 s)

All the excited electronic states of C6H5F+• , C6H5I+•

do not have long lifetimes.

B~

Photoelectron spectraPhotoelectron spectraPhotoelectron spectraPhotoelectron spectra

ⅡⅡ. . Mass-analyzed threshold ionization(MATMass-analyzed threshold ionization(MATII) spectroscopy) spectroscopy

A. PrincipleA. Principle

1) 1) OutlineOutline

Photo-excite a molecule to a Rydberg state (high n) lying just Photo-excite a molecule to a Rydberg state (high n) lying just

below ( < 10cmbelow ( < 10cm-1-1) the ionization limit.) the ionization limit.

Some ions and electrons are generated by direct Some ions and electrons are generated by direct

photoionization (direct ions/electrons). Remove these.photoionization (direct ions/electrons). Remove these.

IIonize the molecule in Rydberg state (Rydberg neutral) byonize the molecule in Rydberg state (Rydberg neutral) by

applying electric field (pulse-field ionization, PFapplying electric field (pulse-field ionization, PFII).).

Scan hScan h. Record spectrum by detecting. Record spectrum by detecting

electrons → Zero electron kinetic energy spectrum (ZEKE). electrons → Zero electron kinetic energy spectrum (ZEKE).

ions → MATions → MATII

2) 2) MATMATII vs. ZEKE vs. ZEKE

Weakness Weakness

Poor resolution [ZEKE : 5cm-1 (conventional), 0.1 cm-1

(high resolution), MATI : 10cm-1], related to removal of

heavy ions compared to removal of e- in ZEKE.

Strength Strength

Identification of ions contributing to each peak.

Generation of state-selected ions.

3) 3) Lifetime of a Rydberg neutralLifetime of a Rydberg neutral

Rydberg states (high n , low ℓ)Rydberg states (high n , low ℓ)

∝ n3

n = 200 → ~ 100 nsecn = 200 → ~ 100 nsec

ZEKE states (high n , ℓ , m )ZEKE states (high n , ℓ , m )

∝ n4

n = 200 → ~ 20 n = 200 → ~ 20 secsec

A successful MATI detects ions from ZEKE states generated by PFI after a long delay time (sec).

B. PhotoexcitationB. Photoexcitation

hIE = 8 ~ 12eV (100 ~ 150nm)

two-photon 1 + 1two-photon 1 + 1 one-photonone-photon

Two-photon MATITwo-photon MATI

Difficult to control multiphoton processes.Difficult to control multiphoton processes.

Applicable to systems with a stable intermediate state with E < 5.6 eV Applicable to systems with a stable intermediate state with E < 5.6 eV = 220nm. For most neutrals, 1st excited states are not stable.= 220nm. For most neutrals, 1st excited states are not stable.

One-photon MATIOne-photon MATI

No complications as above.No complications as above.

Requires vacuum ultraviolet (VUV) laser.Requires vacuum ultraviolet (VUV) laser.

h1

h2

C. C. IInstrumentationnstrumentation

1) 1) VUV laserVUV laser

Four-wave difference frequency mixing in KrFour-wave difference frequency mixing in Kr

h1

h2

h3

h4

4p6

5p[5/2]2

5p[1/2]0

h1 = h2 = 212.6 nm or 216.7 nm

h3 = 400 ~ 800 nm

h4 = 122 ~ 145 nm, 10 nJ

Four-wave sum frequency mixing in HgFour-wave sum frequency mixing in Hg

h1

h2

h3

h4

61S0

71S0

h1 = h2 = 312.8 nm

h3 = 340 ~ 650 nm

h4 = 107 ~ 126 nm, 20 nJ ~ 200nJ

TO

F

MCP

LiF lens

Achromaticlens

UV, S

Temperature-controlledpulsed valve

Hg

Heatingblock

Ar

Water inWater in

Out Out

2) 2) MATMATII spectrometer spectrometer

((a) Top viewa) Top view

dichroic mirrorKr cellMgF2 lens

photoionization chamber

50cm lens

((b) Side viewb) Side view

detector

molecular beam VUVE3

E2E1G

TOF

3) 3) Pulsing schemePulsing scheme

E1

E2

E31200V

950V

photoexcitation

PFI delay

ⅢⅢ. . MATMATII spectra in the ground spectra in the ground electronic stateelectronic state

Photon Energy, cm-1

CC66HH553535ClCl+•+•CC66HH553535ClCl+•+•

CC66HH553737ClCl+•+•CC66HH553737ClCl+•+•Io

n S

ign

al

Ion

Sig

nal

Photon Energy, cm-1

CC66HH557979BrBr+•+•CC66HH557979BrBr+•+•

CC66HH558181BrBr+•+•CC66HH558181BrBr+•+•

Photon Energy, cm-1

CC66HH55II+•+•CC66HH55II+•+•

Photon Energy, cm-1

CC66HH55FF+•+•CC66HH55FF+•+•

IIonization energies (onization energies (IIE) to the ground ( E) to the ground ( 22BB11) and ) and 22BB22 excited states excited states

of chloro-, bromo-, iodo-, and fluorobenzene cations, in eVof chloro-, bromo-, iodo-, and fluorobenzene cations, in eV

Chlorobenzene 9.0728 ± 0.0006 11.3327 ± 0.0006 This work

9.0723 ± 0.0006 MATI

9.0720 ± 0.0006 ZEKE

9.066 ± 0.008 11.330 ± 0.008 PES

Bromobenzene 8.9976 ± 0.0006 10.6406 ± 0.0006 This work

8.991 ± 0.008 10.633 ± 0.008 PES

8.98 ± 0.02 MPI-PES

Iodobenzene 8.7580 ± 0.0006 This work

8.754 ± 0.008 PES

8.77 ± 0.02 PEPICO

Fluorobenzene 9.2033 ± 0.0006 This work

9.2033 ± 0.0006 MATI

9.2044 ± 0.0005 ZEKE

9.18 ± 0.02 MPI-PES

IE( 22BB11) IE( 22BB22) Ref.B~X~

X~ B~

Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for the and their assignments for the

ground state ( ground state ( 22BB11) chlorobenzene cation.) chlorobenzene cation.Mode This work(Wilson) C6H5

35Cl+• C6H5 37Cl+•

972600(?)

415530

1114155415921193

771139710482991

1408829

1246166120782225

950113113601392152718212277

974600(?)

419527

1118155415931193 771141713482991286

1411838

1260167720972235

95011351368

1394 1533 1828 2280

975

422531

1116

1200

716394995311

1429

971

420526

1115

1194

714393992

950

422510

1100

1180

720

960

427

1121

1003685417615

1093158615981153

741 197

706467

1026287

1482

a1

b1

a1

b2

a1

a1

b2

a1

b1

b1

a1

b1

a1

b2

a1

1 46a6b 7a 8a 8b9a 10b1112 16b 18a18b19a6a2

6a3

6a4

6a5 7a2

6a16b1

6a1121

6a26b1 6a111 6a17a1

7a1121 8a1121

symmetry Neutral PES MPI-PES MATI ZEKE

X~

Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for the and their assignments for the

ground state ( ground state ( 22BB11) bromobenzene cation.) bromobenzene cation.X~

3083(?)331593

1073157715231193

791126678

1307396

1008257

14663083(?)

659987

13221653

9281402173417541911206122412474

3083(?) 329 593 1073 1577 1523 1193

791 126 678 1307 394 1008 257 14663083(?) 659 986 1320 1649

1399 1729 1750 1907 2058 2239

2471

Mode This work(Wilson) C6H5

81Br+•

950

320540

11001530

1180

720

980

331

1016

10013065

314614

10701578

11761158

736181671

1321409

1020

14723067

a1

a1 a1

b2

a1

a1

b2

a1

b2

b1

b1

a1

b2

a2

a1

b2

a1

a1

1 26a6b 7a 8a 8b9a 9b10b1112 1416a 18a18b19a20a6a2

6a3

6a4

6a5 6a16b1 6a7a6a27a 7a12 6a8a6a37a 6a28a 6a7a2

symmetry Neutral PES MPI-PESC6H5

79Br+•

Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for and their assignments for the ground state ( the ground state ( 22BB11) iodobenzene cation.) iodobenzene cation.

Mode (Wilson)

990 284 538103615751517 808 127 661 357 406 9031015 242 567 8481129 9431226126912961310154815941648167616952256

331

1016

998 268 61210631575

729 167 654 398 421 9031015 220

a1

a1 b2

a1

a1

b2

b1

b1

a1

a2 b1

b1

a1

b2

1 6a 6b 7a 8a 8b 10b 11 12 16a 16b 17b 18a 18b 6a2

6a3

6a4 6a1121 18b111 6a111

6a118a1

6a17a1 6a211 6a27a1 11121

12118a1 7a1121

6a1121

symmetry Neutral PES This work

X~

Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for and their assignments for the ground state ( the ground state ( 22BB11) fluorobenzene cation.) fluorobenzene cation.

Mode(Wilson)

1299 500 60612741610157411681106 763 182 80413391071 479 402150214641668179718422109196822822343

500 505

1164

181 795

400

500510

1620

1170

810

410

1301 517 61512321604159711561128 754 249 80913261066 498 40015001460

b2 a1

b2 a1 a1

b2

a1

b2

b1

b1

a1

b2

b2

b1

b2

a1

b2

36a6b 7a 8a 8b9a 9b10b1112 141516b 18b19a19b 6a19a1 6a131

6a1141 6a18a1 9a1121 9a19b1 9a2

symmetry Neutral MPI-PES MATI This work

X~

66aann progression progression66aann progression progression

Prominent for C6H5Cl+• , C6H5Br+• , C6H5I+•.

Not so for C6H5F+• . Why?

Calculation of geometrical change upon ionization.

Calculation of mode eigenvectors for ions.

B3LYP / 6-311++G ** ** and other levels.

geometry change upon ionization 6a eigenvector

Photon Energy, cm-1

cmcm-1-1

2B2 , C6H535Cl+• 2B2 , C6H535Cl+•B~

ⅣⅣ. . MATMATII spectra in the Bspectra in the B22BB22 excited electronic state excited electronic state~

Photon Energy, cm-1

cm-1

2B2, C6H579Br+• 2B2, C6H579Br+•B~

Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for and their assignments for the chlorobenzene cation in the the chlorobenzene cation in the 22BB2 2 excited state. excited state.

Mode(Wilson)

9611279 667

382 54610801173

759 153 725 329 439 8991009 246 709 870

1010

730 384 56211311263

636 223 218

866 329

970

340

1003 1271 682 985 420 61610851174 830 740 196 701 400 467 9021026 297

a1

b2 b1 b1

a1

b2

a1

a1

a2

b1

b1

a1

a2

b1

b1 a1

b2

13456a6b 7a 9a 10a10b1112 16a16b17b 18a18b 6a116a1 6b116a1

symmetry Neutral PES REMPDS PIRI This work

869

387

943

761

313

260

B~

Mode(Wilson)

9591251 5421015157111801130 6221333 889 9821419

970

620

10011264 6141070157811761158 6711321 90410201472

a1

b2 b2 a1

a1

a1

b2

a1

b2

b1

a1

a1

136b 7a 8a9a 9b12 1417b 18a19a

symmetry Neutral PES This work

Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for and their assignments for

the bromobenzene cation in the the bromobenzene cation in the 22BB2 2 excited state. excited state. B~

Photon Energy, cm-1

2B2, C6H5 I+• 2B2, C6H5 I+•B~

ⅤⅤ. . Selection ruleSelection rule

TheoreticalTheoretical

Transition moment for the R (Rydberg) ← X (ground) transitionTransition moment for the R (Rydberg) ← X (ground) transition

Born - Oppenheimer Born - Oppenheimer approximationapproximation

Ground stateGround state → zero–point level ( beam condition),∵→ zero–point level ( beam condition),∵totally symmetry (atotally symmetry (a11))

→ → vibrational state of R should be avibrational state of R should be a11 also. also.

aa11 propensity rulepropensity ruleaa11 propensity rulepropensity rule

observationobservation

aa11 > b > b22 > b > b11 >> a >> a22 Why? Why?

~ < ~ < elelRR││││elel

XX > < > < vibvibRR││vibvib

XX > >

RXRX = < = < RR││││ XX > >

ⅥⅥ. . Summary and conclusionSummary and conclusion

1.1. MATMATII spectra of C spectra of C66HH55XX+• +• in the ground ( X = Cl, Br, in the ground ( X = Cl, Br, II, F ) and B, F ) and B22BB22

excited ( X = Cl, Br, excited ( X = Cl, Br, I I ) electronic states obtained by one–photon ) electronic states obtained by one–photon VUV- MATVUV- MATII spectroscopy. spectroscopy.

2.2. Accurate ionization energies and vibrational frequencies in the Accurate ionization energies and vibrational frequencies in the ground ( X = Cl, Br, ground ( X = Cl, Br, II, F ) and B, F ) and B22BB22 excited ( X = Cl, Br ) electronic excited ( X = Cl, Br ) electronic

states determined.states determined.

3.3. The ground state MATThe ground state MATII spectra ( X = Cl, Br, spectra ( X = Cl, Br, II ) display prominent ) display prominent 6a6ann progression due to geometry change upon ionization along progression due to geometry change upon ionization along the 6a eigenvector.the 6a eigenvector.

4.4. Well-resolved vibrational spectra obtained for BWell-resolved vibrational spectra obtained for B22BB22 of C of C66HH55ClCl+•+• and and

CC66HH55BrBr+•+• which are very long-lived states. Broad band spectrum which are very long-lived states. Broad band spectrum

obtained for Bobtained for B22BB22 of C of C66HH55II+•+• which has a short lifetime. which has a short lifetime.

~

~

~

~

5. A routine spectroscopic technique, VUV-MATI, has been developed to record vibrational spectra of polyatomic ions.